HIV-1 encodes a number of genes that are crucial for replication in primate cells. Gag, Pol, and Env products represent the main virion components, while Tat and Rev products regulate intracellular transcriptional and post-transcriptional events for the controlled expression of viral genes. Of particular interest are the HIV accessory proteins Vif, Vpr, Vpu, Vpx, and Nef, which are unique to primate lentiviruses. There is increasing evidence that these proteins operate in conjunction with specific host factors. In fact, most if not all, of the accessory proteins appear to lack catalytic activities but instead seem to function as adaptors to link viral or cellular factors to pre-existing cellular pathways. In FY 2004, we focused primarily on the role of Vif in inhibiting the antiviral activity of the host deaminase APOBEC3G. We completed and published a study demonstrating that the production of infectious HIV-1 does not require depletion of APOBEC3G from virus-producing cells. We further demonstrated and published that viral RNA is required for the association of APOBEC3G with HIV-1 nucleoprotein complexes. The latter study is significant because it established that while APOBEC3G can be packaged non-specifically into virus-like particles, the specific packaging of the deaminase into the viral nucleoprotein complex of infectious virions requires the highly specific interaction of APOBEC3G with a structure in the 5? non-translated region of the genomic RNA. Finally, we conducted and published a study on SIVagm Vif, which demonstrated that Vif can inhibit the activity of virus-associated APOBEC3G. This study is significant as it demonstrates that Vif can inactivate APOBEC3G through several independent mechanisms. Aside from these completed and published studies, we initiated a series of studies to (i) investigate the role of RNA for the oligomerization of APOBEC3G and/or its interaction with Vif; (ii) investigate the role of APOBEC3G for replication of HIV in primary cells; (iii) study the mechanism of APOBEC3G packaging and interference by Vif; (iv) identify dominant-negative mutants of Vif, which will provide important mechanistic insights; (v) to study domain(s) in Vif important for interaction with APOBEC3G and other host factors. From these studies we hope to get a better understanding of the function of Vif with respect to its manipulation of host machineries.